As the degree of integration of semiconductor devices increases, the area occupied by each semiconductor device on a chip also decreases. Accordingly, capacitors, which store information in a dynamic random access memory (DRAM), must be made smaller while still retaining at least the same storage capacity.
There are many studies in progress for increasing the capacity of capacitors, and recently, a technique of using a dielectric film having a high dielectric constant has become one main technique. Dielectric films having a high dielectric constant are frequently metal oxides obtained from a metal having a high oxygen affinity. Such a dielectric film may have a large leakage current since the dielectric constant is not stable. A metal having a high work function may be necessary for the capacitor electrode when using a dielectric film having an unstable high dielectric constant. A capacitor having the above mentioned structure is generally called a metal-insulator-metal (MIM) capacitor.
A tantalum oxide film (Ta205) is often used as a dielectric film of a conventional MIM capacitor. The tantalum oxide film has a dielectric constant of about 25. The tantalum oxide film may generate a large leakage current even though its dielectric constant is high. Accordingly, when using the tantalum oxide film as a dielectric film, ruthenium (Ru) metal is frequently used as the electrode of the capacitor since ruthenium has superior barrier characteristics compared to other metals. However, ruthenium is very expensive, making it less than ideal for mass-production.
A titanium nitride film (TiN) can also be used as an electrode material of a capacitor, since it is less expensive than ruthenium and is often used in semiconductor manufacture. The titanium nitride film has low reactivity, stable leakage current characteristics and superior conductivity. However, since the titanium nitride film has lower barrier characteristics than the ruthenium metal layer, the tantalum oxide film cannot be used as a dielectric film when the titanium nitride film is used as an electrode, due to its large leakage current.
Accordingly, a hafnium oxide (HfO2) film, which has better leakage current characteristics than the tantalum oxide film, is often used as a dielectric film when titanium nitride electrodes are used. The hafnium oxide film has an almost identical dielectric constant of about 20 to 25 and higher reliability than the tantalum oxide film. A capacitor having the structure of titanium nitride film/hafnium oxide film/titanium nitride film is cheaper, generates lower leakage current, and provides higher capacitance than a capacitor having the structure of ruthenium/tantalum oxide film/ruthenium. Therefore, the titanium nitride/hafnium oxide/titanium nitride capacitor is more suitable for a DRAM device with a design rule of less than 100 nm. A technique of using such a hafnium oxide film is disclosed in U.S. Pat. Nos. 6,348,368 B1 and 6,420,279.
However, the structure of a titanium nitride/hafnium oxide/titanium nitride capacitor has some drawbacks. This type of capacitor generates lower leakage current when it is manufactured. After manufacture, however, the capacitor may generate a large leakage current while performing a back-end process of a semiconductor device for forming an interlayer insulating layer (not shown), a barrier metal layer (not shown) and a metal interlayer insulating layer (not shown), especially during a deposition process of a barrier metal layer at a high temperature. By the tight thermal process, the hafnium oxide film is crystallized and then a leakage current is generated in the hafnium oxide film. That is, if the hafnium oxide dielectric film is crystallized during the high thermal process, leakage current is increased by crystal defects generated in the hafnium oxide film.
FIG. 1 is a graph showing a crystal peak of a hafnium oxide dielectric film of a capacitor. In the graph, a curve (a) shows a crystal peak of a hafnium oxide film as deposited, and curves (b), (c) and (d) show crystal peaks of the hafnium oxide film after the back-end process. In more detail, curve (b) shows a crystal peak of the hafnium oxide film processed at 500° C., curve (c) shows a crystal peak of the hafnium oxide film processed at 537° C., and curve (d) shows a crystal peak of the hafnium oxide film processed at 550° C. As shown in the graph, there is no peak beside a crystal peak of a lower electrode made of a titanium nitride film right after forming the capacitor, when using a hafnium oxide film as a dielectric film of a capacitor. However, the dielectric film shows crystal peaks after performing the back-end process. Therefore, crystal defects may be generated in the dielectric film. Also, the hafnium oxide film must be formed 10 nm thick for a DRAM device of less than 100 nm design rule. The hafnium oxide film may be deposited to a uniformed thickness on a flat plane. However, it is very difficult to form the hafnium oxide film to a uniform thickness on the three-dimensional surface of a lower electrode. Therefore, additional leakage current is generated.